1. Field of the Invention
The present invention relates generally to voltage controlled oscillators (VCO), and more particularly, to digitally implemented voltage controlled oscillators controlling an oscillating frequency with an input voltage. The invention further relates to a method of operating such voltage controlled oscillators.
2. Description of the Background Art
A voltage controlled oscillator for converting an analog signal to an output signal having a variable frequency corresponding to that analog signal is used in various circuits such as a radio tuning circuit, and a sound synthesizing circuit for electronic instruments.
FIG. 12 is a circuit diagram of a conventional voltage controlled circuit comprising an analogue circuit. The voltage controlled circuit comprises a differential amplifier 10, a comparator 11, a buffer amplifier 12, an NPN-type transistor 13, resistors 21-30, and a capacitor 31. An input signal V is applied to an input terminal 14, and an output signal F having a frequency corresponding to a voltage of input signal V is obtained from an output terminal 15. Operating voltage of the voltage controlled oscillator is V.sub.DD. Resistance value of resistor 21 is 2R, and resistance value of each of the resistors 22-24 is R. In addition, resistance value of each of the resistors 26, 27, and 29 is R, Capacitance value of capacitor 31 is C.
The operation of the voltage controlled oscillator will be described. At first, when input signal F is in a low level (referred to as "L" hereinafter), transistor 13 is in an off state. The potential of a positive input terminal N2 of comparator 11 is set to (1/3).multidot.V.sub.DD by resistors 26, 27, and 29, since the output of buffer amplifier 12 becomes ground potential. If input signal V having a positive voltage is applied to input terminal 14 when transistor 13 is in an off state, capacitor 31 is charged through resistor 21. Since a negative input terminal N1 of differential amplifier 10 has the fixed potential, the output voltage of differential amplifier 10 decreases. The potential level of output signal F of comparator 11 is inverted into a high level (referred to as "H" hereinafter) when the output voltage becomes lower than the voltage (1/3).multidot.V.sub.DD of positive input terminal N2 of comparator 11.
As a result, transistor 13 turns on, so that the capacitor 31 is discharged and the output voltage of the differential amplifier 10 is increased. Since the output of buffer amplifier 12 becomes power supply potential V.sub.DD, the potential of positive input terminal N2 of comparator 11 is set to (2/3).multidot.V.sub.DD by resistors 26, 27 and 29. When the output voltage of differential amplifier 10 becomes higher than the potential (2/3).multidot.V.sub.DD of positive input terminal N2 of comparator 11, the potential level of output signal F of comparator 11 is inverted into "L", so that the transistor 13 turns off.
In this manner, output signal F having the frequency corresponding to the voltage of input signal V is obtained.
In the voltage controlled oscillator described above, the relationship between an input voltage V.sub.in of input signal V and a frequency f.sub.0 of output signal F will be represented by the following equation: EQU f.sub.0 =(3/8).multidot.(V.sub.in /R.multidot.C.multidot.V.sub.DD) (1)
As can be seen from the above equation (1), in the conventional voltage controlled oscillator comprising the analogue circuit, a voltage-frequency converting characteristic is dependent on operating voltage V.sub.DD, value R of an input resistor and capacitance value C of capacitor 31. In particular, capacitor 31 has the characteristic varied by temperature. That is, all kinds of capacitors have a characteristic which varies depending on a temperature around the capacitor. It is required to control temperature more accurately to obtain more precise oscillation. However, more accurate temperature controlling is difficult to be carried out. Therefore, it is difficult to obtain a stable characteristic, since the converting characteristic is varied by variation of operating voltage and operating temperature.
Although the linear converting characteristic in which input voltage V.sub.in is proportional to frequency f.sub.0 can be obtained, the non-linear converting characteristic cannot be obtained. Furthermore, if the converting characteristics are to be changed, it is required to change resistance value of the resistor or capacitance value of the capacitor.